Multi-piston engine, such as a pump or compressor, with improved vibrational behaviour

ABSTRACT

The invention relates to a piston engine which is essentially comprised of an engine shaft ( 1 ) and a set of modules, each of which comprises a cylinder ( 2   a,    2   b ) and a piston ( 3   a,    3   b ) powered by the engine shaft in the cylinder, each module resulting in vibratory excitation of the engine at a determined fundamental frequency (Fa, Fb). Rather than introducing phase differences between the various vibratory excitations, as is the case in prior art, the invention introduces a frequency offset (ΔFab) between the fundamental frequencies (Fa, Fb) thereof in order to avoid a high-amplitude vibration at a common excitation frequency (F), arising from the spectral spread thus obtained.

[0001] The present invention relates generally to multi-piston engines,such as for pumps or compressors, allowing one to put in motion or topressurize a hydraulic, gaseous or even a polyphase fluid.

[0002] More precisely, the invention relates to a multi-piston engine ofthe type which comprises: a drive shaft; a number of cylinders; a numberof pistons selectively driven by the alternating motion of the driveshaft and respectively housed in the cylinders in order to delimit, fora fluid put in motion by the engine, a corresponding number of chamberswith variable volume, each of which presents a determined maximum volumeto the fluid and has a low pressure intake and a high pressure outlet;and a number of delivery channels connecting the high pressure outletsof the respective chambers to the delivery outlet of the engine, eachdelivery channel having a determined cross section and a determinedlength, the movement of each piston bringing about a vibrationalexcitation of the engine, at a determined fundamental frequency, andeach chamber, in association with the piston which delimits it and withthe delivery channel which connects it to the delivery outlet, forming acorresponding module of the engine.

[0003] Piston engines of this type are well known, particularly throughFrench Patent No. 2 655 690, and are very widely used, particularly onmotor vehicles for powering assisted hydraulic circuits such as ABSbraking systems, automatic transmissions, power steering or suspensions,particularly hydropneumatic suspensions.

[0004] Although there are a number of sub-categories of piston enginesof this type, which are represented in particular by the radial pistonengines and the engines with axial pistons and tilting plate, the designof these piston engines always runs into the problem of reducing thenoise and more generally the vibrations produced during operation.

[0005] French Patent No. 2 551 505 describes, for example, a pumpingsystem for liquid phase chromatography which aims to reduce theinstantaneous flow rate variations and which can possibly obtain acertain reduction in operating noise, at least as an incidental result.

[0006] Such a system nevertheless requires both the use of cams with aprofile in the form of an arithmetic spiral and of cylinders with greatdimensional differences between one another, where such structuralconstraints are very difficult to accept for engines for industrial usethat are mass-produced.

[0007] FR 1 546 997 and DE 196 41 779 describe piston engines forindustrial use which are primarily designed to operate with a reducednoise level.

[0008] The basic principle used in these known engines consists ofarranging the cylinders in an irregular manner, so as to introduce anangular offset between the individual points of their respectiveoperating cycles and to attenuate the resulting noise by shifting thephase between the different elementary noises which are emitted.

[0009] Nevertheless, this irregular arrangement of the cylinders makesmachining of such engines relatively delicate and expensive.

[0010] In this context, the invention aims to propose a piston enginewhich has clearly improved vibrational behavior, without requiringextensive structural modifications of structure with respect to standardpiston engines.

[0011] To this end, the piston engine of the invention, which otherwiseconforms to the generic definition stated in the introduction above, isessentially characterized by the fact that it has at least a firstspectral spreading means associated with a first pair of modulesincluding a first pair of pistons, where this first spectral spreadingmeans is suitable for introducing, between the fundamental frequenciesof the vibrational excitations resulting from the respective movementsof the pistons of this first pair, a frequency shift equal to no morethan 10% of the fundamental frequency of the vibrational excitationresulting from the movement of either piston of this first pair.

[0012] Thus, instead of seeking to phase-shift the noise emitted by thedifferent modules, the invention solves the problem of noise reductionthanks to a relatively modest modification of the frequencies emitted bythe different modules.

[0013] Preferably, the first spectral spreading means entails adifference between magnitudes respectively associated with the modulesof the first pair of modules, each of these magnitudes beingrepresented, for the associated module, by the ratio of the crosssection of the delivery channel of this module to the product of themaximum volume of the chamber of this module and the length of thedelivery cannel of this module.

[0014] In order to attain satisfactory effectiveness, the frequencyshift introduced between the fundamental frequencies of the vibrationalexcitations resulting from the respective movements of the pistons ofthe first pair is advantageously equal to at least 1% of the fundamentalfrequency of the vibrational excitation resulting from the movement ofeither of said pistons.

[0015] Excellent results can be obtained by introducing, between thefundamental frequencies of the vibrational excitations resulting fromthe respective movements of the pistons of the first pair, a frequencyshift on the order of 2% of the fundamental frequency of the vibrationalexcitation resulting from the movement of either of said pistons.

[0016] The invention thus allows the pistons which respectively delimitthe chambers of the modules of the first pair of modules to beidentical.

[0017] The first spectral spreading means can, for example, entail atleast one difference between the maximum volumes of the respectivechambers of the modules of the first pair of modules.

[0018] For this purpose, it is possible to provide different cylindersof the modules of the first pair of modules, for example, and which aremachined in such a way as to have different lengths or diameters, wherethese differences, however, are then preferably provided outside of theregions in which the pistons move so that the pistons can remainunchanged and identical.

[0019] However, the cylinders of the modules of the first pair ofmodules can also be identical; at least one of these cylinders can thencontain a solid filler block which modifies the maximum volume of thechamber defined by this cylinder.

[0020] Such a filler block is, for example, formed by a stack of severalblock elements all having the same volume and produced out of a flexiblematerial such as polyurethane or a compressible elastomer.

[0021] The engine of the invention can have as many spectral spreadingmeans as the number of pairs of modules containing neighboring pistons,and even possibly as many spectral spreading means as the number ofmodules.

[0022] In the engine of the invention, the cylinders can thus bearranged in a regular manner with respect to the drive shaft.

[0023] The advantages provided by the invention are particularlysignificant when each delivery channel is separated from the deliveryoutlet of the piston engine by a non-return valve.

[0024] Other characteristics and advantages of the invention will emergeclearly from the description given below, on an indicative and in no waylimiting basis, in reference to the appended drawings in which:

[0025]FIG. 1 is a cross section of a piston engine of the type to whichthe invention is applicable; and

[0026]FIG. 2 is an enlarged schematic view of an engine module accordingto the invention.

[0027] As shown in FIG. 1, the invention relates to a piston engine,which in this case constitutes a hydraulic pump, and which essentiallyhas drive shaft 1, cylinders, such as 2 a, 2 b, pistons, such as 3 a, 3b, which, in the cylinders, define chambers, such as 4 a, 4 b, deliveryoutlet 6 of the engine, and delivery channels, such as 5 a, 5 bconnecting the chambers to delivery outlet 6.

[0028] Inasmuch as, for comprehension of the invention, the number ofcylinders and pistons can be assumed to be any number, and particularlyan even or odd number, provided that it is at least equal to two, theassumption will be made that the piston engine can contain at least onecylinder 2 c, one piston 3 c, one chamber 4 c, and one delivery channel5 c in addition to those illustrated by FIG. 1.

[0029] Pistons 3 a, 3 b, 3 c are given an alternating movement at willby drive shaft 1, inside of cylinders 2 a, 2 b, 2 c, for example, bymeans of cam 11, so that chambers 4 a, 4 b, 4 c have a variable volume,between a minimum volume that is as small as possible and a maximumvolume, noted Va, Vb, Vc respectively, for chambers 4 a, 4 b, 4 c.

[0030] Each chamber such as 4 a, 4 b, 4 c has a low pressure intake suchas 40 a, 40 b, 40 c and a high pressure outlet such as 41 a, 41 b, 41 cconnected to delivery outlet 6 of the piston engine throughcorresponding delivery channel 5 a, 5 b, 5 c, at the end of whichassociated non-return valve 7 a, 7 b, 7 c is installed, which prohibitscirculation of fluid from outlet 6 of the piston engine towards thecorresponding chamber.

[0031] Each delivery channel such as 5 a, 5 b, 5 c has a determinedcross section Sa, Sb, Sc and a determined length La, Lb, Lc.

[0032] Each chamber, such as 4 a, 4 b or 4 c, in association with piston3 a, 3 b, or 3 c which delimits it in a cylinder such as 2 a, 2 b, or 2c, and in association with delivery channel 5 a, 5 b, or 5 c whichconnects this chamber to delivery outlet 6, forms a corresponding moduleof the engine, such as module 3 a, 4 a, 5 a illustrated in FIG. 2.

[0033] In practice, in the prior art, the chambers all have the samemaximum volume V, and the delivery channels all have the same crosssection S and the same length L.

[0034] Under these conditions, the movement of each piston 3 a, 3 b, 3 cbrings about a vibrational excitation of the piston engine, whosefundamental frequency F is given by the equation:

F=(λ·S/4π² ·ρ·V·L)^(½)

[0035] where λ is the coefficient of compressibility of the pumpedfluid, and ρ is the specific gravity of this fluid.

[0036] The total excitation exerted on the piston engine during onerevolution of drive shaft 1, and which is represented by the result ofthe excitations exerted by the movement of the different pistons, canthus, in the prior art, reach a considerable amplitude at the commonexcitation frequency F of the different modules of the piston engine.

[0037] In order to solve this problem, the invention provides a certainspectral spreading of the excitation frequencies of the different unitmodules of the piston engine, which is obtained by introducing, for atleast two different modules, for example, 3 a, 4 a, 5 a, on the onehand, and 3 b, 4 b, 5 b, on the other, a frequency shift ΔFab betweenthe fundamental frequencies Fa, Fb of the vibrational excitationsbrought about by pistons 3 a and 3 b of these two modules, that is, bymaking these frequencies Fa and Fb different.

[0038] More precisely, the frequency shift ΔFab thus introduced is,according to the invention, chosen to be equal to up to 10% of thefundamental frequency Fa of the vibrational excitation resulting fromthe movement of either of the pistons of these two modules, for example,of piston 3 a.

[0039] In order to give the invention its greatest effectiveness, thefrequency shift ΔFab introduced between the fundamental frequencies Fa,Fb of the vibrational excitations brought about by the given pistons oftwo modules, such as 3 a, 4 a, 5 a and 3 b, 4 b, 5 b, is chosen to be atleast equal to 1% of the fundamental frequency Fa of the vibrationalexcitation brought about by piston 3 a of any one of these modules, andis optimally chosen to be on the order of 2% of this fundamentalfrequency Fa.

[0040] In order to better understand the possibilities offered by theinvention, it is convenient for each module, such as 3 a, 4 a, 5 a or 3b, 4 b, 5 b, to define a corresponding magnitude, such as Ga or Gb,given by:

Ga=Sa/(Va·La) and Gb=Sb/(Vb·Lb).

[0041] In other words, for each module, this magnitude, such as Ga orGb, is represented by the ratio of the cross section, Sa or Sb, of thedelivery channel, 5 a or 5 b, of this module, to the product of themaximum volume, Va or Vb, of chamber 4 a or 4 b of this module, and thelength, La or Lb, of delivery channel 5 a or 5 b of this module.

[0042] Under these conditions, the desired spectral spreading betweenthe fundamental frequencies Fa, Fb of the vibrational excitationsrespectively attributable to two modules such as 3 a, 4 a, 5 a and 3 b,4 b, 5 b, is obtained by making the magnitudes Ga and Gb respectivelyassociated with these modules different, which can be obtained byintroducing a difference in the respective maximum volumes Va and Vb ofthe chambers of these modules, or in the respective cross sections Sa,Sb of delivery channels 5 a, 5 b of these modules, or in the respectivelengths La, Lb of these delivery channels 5 a, 5 b, or else in severalof these parameters at the same time, insofar as the effects of suchmodifications relating to several magnitudes at the same time do notcompensate for one another, and that the magnitudes Ga and Gb aretherefore indeed different from one another.

[0043] Preferably, the frequency shift intended for ensuring the desiredspectral spreading is introduced between the fundamental frequencies ofthe vibrational excitations is brought about by each pair of moduleswhich contain neighboring pistons.

[0044] Thus, if the piston engine contains an even number of modulesarranged in a circle, it is preferable to make the excitationfrequencies resulting from the functioning of two neighboring modulesdifferent, which can be brought about with a minimum of two differentexcitation frequencies.

[0045] If, for example, the piston engine contains four modules, thefirst of which is designated 3 a, 4 a, 5 a, the second 3 b, 4 b, 5 b,the third 3 c, 4 c, 5 c, and the fourth 3 d, 4 d, 5 d, and if thesemodules are arranged in a circle successively in this order, theinvention can be implemented by making Fb≠Fa and Fd≠Fc, even if Fc=Faand Fd=Fb.

[0046] Nevertheless, it can prove to be advisable to control eachexcitation frequency to make ensure that the excitation frequencies areall different from one another.

[0047] For a frequency Fa between 800 and 1000 Hz, the frequency shiftΔFab can be chosen so as typically to be on the order of 20 Hz.

[0048] Since the parameters Sa, Va and La which define each magnitudesuch as Ga=Sa/(Va·La) do not include the diameter of the cylinders suchas 2 a, it is possible to implement the invention while using pistons,such as 3 a, which are identical for all the modules.

[0049] Likewise, the cylinders such as 2 a, 2 b, 2 c can be arranged ina regular manner with respect to drive shaft 1, and for example, aroundthis drive shaft, instead of having to be distributed in an irregularmanner, as is the case in FR 1 546 997 and DE 196 41 779 mentionedabove.

[0050] A simple means of implementing the invention can consist ofintroducing a difference, such as ΔVab, between the maximum volumes,such as Va and Vb, of the chambers, such as 4 a and 4 b, of thedifferent modules such as 3 a, 4 a, 5 a and 3 b, 4 b, 5 b.

[0051] For this purpose, two methods can be considered in particular,the first consisting of machining the cylinders, in which the chambersare defined, in such a way that they are different from one another.

[0052] In order to be able to use identical pistons that work overidentical stroke lengths, it is then preferable to modify the cylindersonly in the dead volumes, that is, in the parts of the cylinders whichare not reached by the pistons and which define the minimum volumes ofthe chambers.

[0053] The second way, which is even more advantageous, consists ofusing identical cylinders, and of placing, in each of the cylinders ofthe modules whose fundamental frequency must be modified, a solid fillerblock, such as 8 a (FIG. 2), which has the effect of modifying themaximum volume Va of chamber 4 a defined in this cylinder 2 a.

[0054] Preferably, filler block 8 a is formed by a stack of severalblock elements of the same volume, such as 80 a.

[0055] Finally, this filler block 8 a is advantageously produced from aflexible material, such as polyurethane or a compressible elastomer.

1. A multi-piston engine, such as, for example, a pump or a compressor,which comprises drive shaft (1); a number of cylinders (2 a, 2 b, 2 c);a number of pistons (3 a, 3 b, 3 c) selectively driven by thealternating motion of the drive shaft and respectively housed incylinders (2 a, 2 b, 2 c) in order to delimit, for a fluid put in motionby the engine, a corresponding number of chambers (4 a, 4 b, 4 c) withvariable volume, each of which presents a predetermined maximum volume(Va, Vb, Vc) to the fluid and has low pressure intake (40 a, 40 b, 40 c)and high-pressure outlet (41 a, 41 b, 41 c); and a number of deliverychannels (5 a, 5 b, 5 c) connecting the high pressure outlets of therespective chambers to delivery outlet (6) of the engine, each deliverychannel (5 a, 5 b, 5 c) having a predetermined cross section (Sa, Sb,Sc) and a predetermined length (La, Lb, Lc), the movement of each piston(3 a, 3 b, 3 c) bringing about a vibrational excitation of the engine,of determined fundamental frequency (Fa, Fb, Fc), and each chamber (4 a,4 b, 4 c), in association with piston (3 a, 3 b, 3 c) which delimits itand with delivery channel (5 a, 5 b, 5 c) which connects it to deliveryoutlet (6), forming a corresponding module (3 a, 4 a, 5 a; 3 b, 4 b, 5b; 3 c, 4 c, 5 c) of the engine, characterized by the fact that itcomprises at least a first spectral spreading means associated with afirst pair of modules (3 a, 4 a, 5 a; 3 b, 4 b, 5 b) including a firstpair of pistons (3 a, 3 b), this first spectral spreading means beingsuitable for introducing, between the fundamental frequencies (Fa, Fb)of the vibrational excitations resulting from the respective movementsof pistons (3 a, 3 b) of this first pair, a frequency shift (ΔFab) equalto at most 10% of the fundamental frequency (Fa) of the vibrationalexcitation resulting from the movement of either (3 a) of the pistons ofthis first pair (3 a, 3 b).
 2. The multi-piston engine according toclaim 1, characterized by the fact that the first spectral spreadingmeans entails a difference (Ga−Gb) between magnitudes (Ga, Gb)respectively associated with the modules of the first pair of modules (3a, 4 a, 5 a; 3 b, 4 b, 5 b), each of these magnitudes (Ga, Gb) beingrepresented, for the associated module, by the ratio (Sa/(Va·La),Sb/(Vb·Lb)) of the cross section (Sa, Sb) of delivery channel (5 a, 5 b)of this module to the product of the maximum volume (Va, Vb) of chamber(4 a, 4 b) of this module and the length (La, Lb) of delivery channel (5a, 5 b) of this module.
 3. The multi-piston engine according to claim 1or 2, characterized by the fact that the frequency shift (ΔFab)introduced between the fundamental frequencies (Fa, Fb) of thevibrational excitations resulting from the respective movements ofpistons (3 a, 3 b) of the first pair is advantageously at least equal to1% of the fundamental frequency (Fa) of the vibrational excitationresulting from the movement of either (3 a) of these pistons.
 4. Themulti-piston engine according to claim 3, characterized by the fact thatthe frequency shift (ΔFab) introduced between the fundamentalfrequencies (Fa, Fb) of the vibrational excitations resulting from therespective movements of pistons (3 a, 3 b) of the first pair is on theorder of 2% of the fundamental frequency (Fa) of the vibrationalexcitation resulting from the movement of either (3 a) of these pistons.5. The multi-piston engine according to any one of the preceding claims,characterized by the fact that pistons (3 a, 3 b) respectivelydelimiting chambers 4 a, 4 b) of the modules of the first pair ofmodules are identical.
 6. The multi-piston engine according to any oneof the preceding claims is combination with claim 2, characterized bythe fact that the first spectral spreading means entails at least adifference (ΔVab) between the maximum volumes (Va, Vb) of the respectivechambers (4 a, 4 b) of the modules of the first pair of modules (3 a, 4a, 5 a; 3 b, 4 b, 5 b).
 7. The multi-piston engine according to claim 6,characterized by the fact that the cylinders (2 a, 2 b) of the modulesof the first pair of modules are different.
 8. The multi-piston engineaccording to any one of claims 1 to 6, characterized by the fact thatcylinders (2 a, 2 b) of the modules of the first pair of modules areidentical, and by the fact that at least one (2 a) of these cylinderscontains a solid filler block (8 a) which modifies the maximum volume(Va) of chamber (4 a) defined in this cylinder (2 a).
 9. Themulti-piston engine according to claim 8, characterized by the fact thatfiller block (8 a) is formed by a stack of several block elements (80 a)all having the same volume.
 10. The multi-piston engine according toclaim 8 or 9, characterized by the fact that filler block (8 a) isproduced out of a flexible material such as polyurethane or acompressible elastomer.
 11. The multi-piston engine according to any oneof the preceding claims, characterized by the fact that is has as manyspectral spreading means as the number of pairs of modules containingneighboring pistons.
 12. The multi-piston engine according to any one ofclaims 1 to 10, characterized by the fact that it comprises as manyspectral spreading means as the number of modules.
 13. The multi-pistonengine according to any one of the preceding claims, characterized bythe fact that cylinders (2 a, 2 b, 2 c) are arranged in a regular mannerwith respect to drive shaft (1).
 14. The multi-piston engine accordingto any one of the preceding claims, characterized by the fact that eachdelivery channel (5 a, 5 b, 5 c) is separated from delivery outlet (6)of the piston engine by non-return valve (7 a, 7 b, 7 c).